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Abstract

Current adaptive optics flood-illumination retina cameras operate at low frame rates, acquiring retinal images below seven Hz, which restricts their research and clinical utility. Here we investigate a novel bench top flood-illumination camera that achieves significantly higher frame rates using strobing fiber-coupled superluminescent and laser diodes in conjunction with a scientific-grade CCD. Source strength was sufficient to obviate frame averaging, even for exposures as short as 1/3 msec. Continuous frame rates of 10, 30, and 60 Hz were achieved for imaging 1.8, 0.8, and 0.4 deg retinal patches, respectively. Short-burst imaging up to 500 Hz was also achieved by temporarily storing sequences of images on the CCD. High frame rates, short exposure durations (1 msec), and correction of the most significant aberrations of the eye were found necessary for individuating retinal blood cells and directly measuring cellular flow in capillaries. Cone videos of dark adapted eyes showed a surprisingly rapid fluctuation (~1 Hz) in the reflectance of single cones. As further demonstration of the value of the camera, we evaluated the tradeoff between exposure duration and image blur associated with retina motion.

Figures (14)

(Left) Layout of the adaptive optics retina camera. The camera consists of three sub-systems: (1) AO for correction of ocular aberrations, (2) pupil retro-illumination and fixation channels for alignment of the subject’s eye, and (3) retinal imaging using a scientific-grade CCD and flood illumination light sources consisting of an SLD and laser diode cascaded with multimode fibers. Details of the camera are included in the text. (Inset) SLD light launched into the multimode fiber is distributed among the fiber modes that propagate along the fiber length at different velocities and reduce the spatial coherence of the SLD light.

Measured RMS wavefront error traces with and without a real-time software filter that suppresses erroneous SHWS measurements, such as those caused by eye blinks. Note the stability of the RMS wavefront error immediately following each blink (as indicated by the black arrows) when the filter is employed compared to when it is not.

Temporal sequence for rapidly collecting (top) four and (bottom) eight images realized by temporarily storing the images on the CCD array. After each exposure, the electron charge on the CCD was rapidly shifted down columns to an unexposed (masked) region of the array in less than 100 µsec. This process was repeated after each additional exposure with the last exposure followed by a read out of the entire CCD array at a rate that minimized read noise.

Average RMS wavefront error across a 6.8-mm pupil, measured in three subjects, with (dark gray) and without (light gray) AO compensation. RMS wavefront error is shown for the total aberrations (2nd through 10th order), Zernike defocus (C4), two Zernike astigmatism modes (C3 and C5), and higher order aberrations (3rd through 10th order). Error bars represent ± one standard deviation from the mean.

Images of approximately the same patch of cone photoreceptors in one subject’s eye (left) with and (right) without the SLD beam passing through the 25 m multi-mode optical fiber. Note the absence of speckle in the right image allowing the cone mosaic to be easily observed.

Individual raw conventional flood illuminated images of the cone mosaic centered at 1.25 deg. eccentricity in one subject’s eye (left) without and (right) with adaptive compensation. For both images, best correction of defocus and astigmatism was achieved with trial lenses and axial translation of the science CCD camera. The 1 deg patch of retina was illuminated by the 679 nm SLD after passing through the 25 m multimode fiber.

Raw 10 Hz flood illuminated video of the cone mosaic in one subject before and during adaptive compensation. The video was captured at 10 Hz with adaptive compensation occurring simultaneously at 15 Hz. The video runs at 10 Hz. The illumination patch subtends 1.8° at 1.4° eccentricity. Exposure duration is 2 msec. Illumination was provided by the 670 nm laser diode after passing through the 300 m multimode fiber. (1.1 MB)

Four-burst videos (top) without and (bottom) with adaptive compensation of a network of retinal capillaries at 1.4° eccentricity in subject RJ. The size of the retinal patch is 1 by 1/2 deg. Both videos were captured at 500 Hz using a 1 msec exposure and 1 msec delay. The videos play at 8 Hz, which is 62.5 times slower than the actual acquired rate. Due to the brevity of the videos, they are best viewed in loop mode in which the video automatically cycles. (2.3 MB)

Eight-burst video with adaptive compensation that shows a network of retinal capillaries. Retinal eccentricity is 1.6°. The size of the retinal patch is 1 by 1/4 deg. Video was captured at 500 Hz using a 1 msec exposure and 1 msec delay. The video plays at 8 Hz. (0.5MB)

(left) Radially-averaged power spectra of the same proximal patch of cones in 20 consecutive video frames for exposure durations of 1/3, 1, 4, 10, 20, 33, 66, and 100 msec. (right) Power ratio is shown averaged across the two subjects and for each of the examined exposure durations. Ratio is defined as power for the 4 msec exposure divided by that for a given exposure. The ratio quantifies the relative benefit of the 4 msec exposure.